"Shadow Walker" is a G blues with an interesting turn around that descends from the V to the IV chord with an added bIII chord at the end for a little more rock! The trick on this one is to keep the riff strong and driving yet laid-back and in the pocket. I can't stress enough listening to the original tune by Free with the great Paul Kossoff on guitar and Paul Rogers on vocals.
The riff is based upon a minor pentatonic scale although the overall tonality of the song is dominant, meaning that we're really hearing G7 as the I7 chord as opposed to G minor. By not actually sounding the natural third, using the flat third instead and slightly bending each time we come across it (I call this "tweaking the third"), the tonality becomes somewhat ambiguous which is a hallmark of the blues. We'll see this tweaked third in almost every song we play in this course. In fact, we'll see a tweaked third in just about every bluesy song we play. Almost by definition that is what makes it bluesy!